An FM receiving circuit is generally equipped with a muting circuit for improvement of stereophonic audio signals in the S/N ratio, and the muting circuit is responsive to a voltage signal supplied from a level detecting circuit provided for detecting an envelope of an intermediate frequency signal produced from a tuned FM signal. The FM receiving circuit is further equipped with a switching circuit which is operative to shift the mode of operation between a stereophonic mode and a monaural mode depending upon the level of the FM signal for improvement in the S/N ratio, thereby preventing the audio sounds from noises. The shifting function is sometimes referred to as "high-blend function".
A typical example of the FM receiving circuit is illustrated in FIG. 1 of the drawings and comprises a front end section 1 coupled to an antenna terminal 2, an intermediate frequency signal amplifier circuit 3, an FM detector circuit 4, a muting circuit 5, an FM demodulation circuit 6 associated with output terminals L and R, a level detecting circuit 7, and a low-pass filter circuit 8 provided with a resistor 9 and a capacitor 10. When the FM receiving circuit is activated, FM signals are received and supplied from the antenna terminal 2 to the front end section 1. The front end section 1 is tuned in a selected frequency range, and, accordingly, the FM signal in the selected frequency range is extracted from the received broadcasting signals. The FM signal is converted into an intermediate frequency signal which is abbreviated as "IF signal", and, then, the intermediate frequency signal is supplied to the intermediate frequency signal amplifier circuit 3. The output of the intermediate frequency signal amplifier circuit 3 is supplied to the FM detecting circuit 4 for demodulation into an FM composite signal. The intermediate frequency signal amplifier circuit 3 is provided with a plurality of amplifier circuits coupled in cascade, and the output signal of each amplifier circuits is detected by the level detecting circuit 7 provided in association therewith, then being summed again. Then, the level detecting circuit 7 produces a voltage signal which traces an envelope of the intermediate frequency signal. In other words, the voltage signal is representative of the electric field strength of the FM broadcasting signal received. The level detecting circuit 7 is further operative to rectify the intermediate frequency signal, and, for this reason, the audio signal is slightly demodulated. As a result, the voltage signal is varied in voltage level by not only the electric field strength of the FM broadcasting signal received but also the audio signal component. On the other hand, the muting circuit 5 starts on the muting operation when a control signal supplied to a control node 5-1 is decreased in voltage level below a predetermined level, and, for this reason, the FM composite signal is blocked by the muting circuit 5. Then, no FM composite signal is supplied to the stereo demodulation circuit 6. When the signal supplied to a control node 6-1 is decreased in voltage level below a predetermined level, the stereo demodulation circuit 6 is operative to gradually decrease the stereo separation factor between a right channel signal and a left channel signal by controlling the high cut off frequency. Thus, the stereo demodulation circuit 6 finally shifts the mode of operation from a stereophonic mode to a monaural mode. If the voltage signal produced by the level detecting circuit 7 is directly supplied to the control nodes 5-1 of the muting circuit 5 as the control signal, the voltage signal tends to deviate from the envelop of the intermediate frequency signal due to the audio signal component contained therein, which sometimes results in unintentional muting functions. Similarly, if the voltage signal is directly supplied to the control node 6-1 of the stereo demodulation circuit 6 as the control signal, the stereo demodulation circuit 6 is liable to decrease the stereo separation factor due to the unintentional decrement in the voltage level due to the audio signal component. In order to avoid these undesirable operation, the low pass filter circuit 8 is coupled between the level detecting circuit 7 and the control nodes 5-1 and 6-1. The low pass filter circuit 8 aims at elimination of the audio signal component from the voltage signal supplied from the level detecting circuit 7, thereby producing a voltage signal which is free from the audio signal component. In other words, the low pass filter circuit 8 is provided for allowing the voltage signal to vary substantially depending upon the electrical field strength only.
However, a problem is encountered in the prior-art FM receiving circuit due to the provision of the low pass filter circuit as follows. When a radio provided with the FM receiving circuit illustrated in FIG. 1 is installed in a vehicle and, then, passes through a shady street or a tunnel, the voltage signal produced in the level detecting circuit 7 is rapidly decreased in voltage level due to reduction of the FM broadcasting signal in the electric field strength. Since the low pass filter circuit 8 is large in time constant for the elimination of the audio signal component, and, for this reason, the voltage signal rapidly decreased in voltage level is rendered slack. The slack voltage signal is supplied to the muting circuit 5 and the stereo demodulation circuit 6 as the control signals and, accordingly, retards the functions thereof. This results in unpleasant noises.